It has been known in the prior art to utilize switching regulators/amplifiers in applications such as, but not limited to: (1) voltage regulators utilized for supplying a relatively fixed DC voltage to a load whose current demands change very quickly such as CMOS logic processors whose activity can go from negligible (such as in standby) to very high or vise-versa in a few nanoseconds, for example, at the change of state of a control signal; and (2) “digital” amplifiers or programmable regulators where the load is relatively fixed but its voltage is changed very rapidly in response to an external command, such as DSL line drivers and supplies or modulators for communication transmitters where the power level or information signal level is changed often and abruptly over a wide dynamic range.
It is also noted that the foregoing applications are characterized by a step down operation where the supply voltage is relatively fixed or slowly varying (such as, for example, a battery), and the widely varying load current is sourced at a voltage that is either fixed or varying, but at a lower value than the supply voltage.
Prior art designs for switching regulators/amplifier to be utilized in the foregoing applications have generally included buck topology switching regulators having low value inductors, high switching frequencies and hysteretic control algorithms without loop filters to achieve high load current slew rates. As is known:
      (                            ⅆ          I                          ⅆ          t                    ∝                        Vin          -          Vout                L              )    .However, the use of such low value inductors results in large values of ripple current and conduction losses, while high switching frequencies result in larger switching losses, both of which undesirably lower efficiency.
In an effort to satisfy performance requirements, it has been known in the prior art to add a cascaded linear amplifier/low drop out regulator immediately before the load, even though the losses due to the load current at the required voltage overhead of the linear stage can be large. Such prior art systems are described, for example, in U.S. Pat. Nos. 4,378,530 and 5,905,407. FIG. 1 illustrates an exemplary block diagram of such a device.
Referring to FIG. 1, the device includes a programmable switching regulator 12 cascaded with a linear amplifier stage 14. In addition, the device includes overhead voltage reference supply 16, and resistors R1 and R2, which are coupled in series to one another and to the output node, VO. The overhead voltage reference supply 16 causes VR=VO+VB1, which is necessary for the linear amplifier to operate, as VR must be larger than VO by an “overhead voltage”. Resistors R1 and R2 form a voltage divider circuit, and provide a feedback signal to the linear amplifier stage 14. The output of the linear amplifier stage 14 operating in conjunction with the output of the programmable switching regulator 12 generate the output voltage, VO, of the device, which is coupled to the load (e.g., a power amplifier in a cell phone application). VSUPPLY corresponds to the voltage source for the device (e.g., a battery in a cell phone application), and VREF sets the output voltage needed to supply the power level required by the load. It is noted that in some applications, VREF will represent the instantaneous power requirement of the load and will include content data (e.g., voice or data information to be transmitted) which is superimposed on the VREF signal utilizing any suitable modulation technique. In operation, the linear amplifier stage 14 essentially functions as the power supply regulator operative to generate a substantially clean signal, VO, which is representative of the instantaneous power required for the task currently at hand.
However, if the output voltage of the switching regulator cannot change rapidly enough to follow voltage changes in VREF, then VR must be set to the instantaneous peak value of VO plus enough additional voltage margin B so that the linear amplifier does not “clip” on signal peaks. If the supply voltage, VSUPPLY, is significantly greater than VR, use of the switching regulator saves most of the power equal to ILOAD*(VSUPPLY−VR), which would otherwise be dissipated in the linear amplifier.
While these known prior art devices provide for an improvement in efficiency, for example, by allowing for a reduction in the switching frequency of the switching regulator, due to the requirements of today's applications and the continued demand for reducing power requirements so as to extend battery life, a further increase in the overall operating efficiency of switching regulators/amplifiers is necessary. It is an object of the present invention to satisfy these needs.